As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and operating voltage, long cycle lifespan, and low self-discharge rate, are commercially available and widely used.
In addition, as interest in environmental problems is recently increasing, research into electric vehicles (EVs), hybrid EVs (HEVs), and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway. As a power source of EVs, HEVs, and the like, a nickel metal-hydride secondary battery is mainly used. However, research into lithium secondary batteries having high energy density, high discharge voltage and output stability is actively underway and some lithium secondary batteries are commercially available.
A cathode, anode, separator and electrolyte of lithium secondary batteries include a variety of additives as power generation elements. Among such additives, an electrolyte additive for lifespan improvement has lifespan extension effect by forming a solid electrolyte interface (SEI) film over a surface of an electrode during an initial battery formation process or to recover an SEI layer partially damaged by a repeated charge/discharge.
The properties of the SEI layer depends on a type of a solvent, properties of additives, and the like included in an electrolyte. In addition, the SEI layer property affects transfer of ions and electric charge, and thereby battery performance may be changed (See. Shoichiro Mori, Chemical properties of various organic electrolytes for lithium rechargeable batteries, J. Power Source (1997) Vol. 68).
When a carbon-based material is used as an anode active material, oxidation/reduction potential is 0.1 V lower than the potential of Li/Li+. Accordingly, a non-aqueous electrolyte is decomposed on an anode surface and thereby the decomposed non-aqueous electrolyte reacts with lithium. The additive also easily reacts and thereby an SEI layer is formed.
The formed SEI layer functions as an ion tunnel and thereby merely lithium ions pass therethrough. Due to such ion tunnel effect, solvation of lithium ions is performed and thereby, among electrolytes, organic solvents, which have a large molecular weight, such as a lithium salt, EC, DMC, DEC, or the like, transferring with lithium ions are inserted into an anode, and, accordingly, disruption of an anode structure may be prevented. In addition, when the SEI layer is formed, lithium ions no longer side react with an anode active material or other materials and quantity of electric charge consumed for formation of the SEI layer as irreversible quantity does not reversibly react when discharged. Therefore, an electrolyte is no longer decomposed and the amount of lithium ions in an electrolyte is reversibly maintained, and, as such, stable charge/discharge may be maintained (J. Power Sources (1994) 51:79to104). As a result, when the SEI layer is formed, the amount of lithium ions is reversibly maintained and, as such, battery lifespan characteristics are also improved.
Accordingly, a variety of electrolyte additives has been introduced to minimize degradation of carbon-based anodes by forming and recovering the SEI layer.
Meanwhile, demand for a battery which may be charged at a high-speed are increasing and thereby interest in use lithium titanium oxide (LTO) as an anode active material is increasing. LTO has advantages such as structural stability and relatively satisfactory cycle characteristics. However, anode materials using LTO as a main active material generate gas such as H2 by catalysis and, as such, lifespan degradation is exhibited.
To address the above problems, attempts have been made to form the SEI layer over a surface of an LTO electrode. However, an anode of lithium secondary batteries including LTO as an anode active material has oxidation/reduction potential that is relatively approximately 1.5 V higher than the potential of Li/Li+ and thereby it is difficult to form the SEI layer with the additive in an SEI layer formation process. Accordingly, battery lifespan performance deterioration problem due to H2 gas generated by continuous catalysis still exists.
Therefore, there is an urgent need for technology which may prevent gas generation by catalysis in a battery using LTO as an anode active material and thereby may improve lifespan characteristics of a battery.